A. Field of Invention
This invention pertains to a method and apparatus for applying cardiac stimulation using multiple electrodes, and more particularly, to a method and apparatus for treatment of congestive heart failure.
B. Description of the Prior Art
The heart is a mechanical pump that is stimulated by electrical impulses. The mechanical action of the heart results in the flow of blood. During a normal heartbeat, the right atrium (RA) fills with blood from the returning veins. The RA then contracts and this blood is moved into the right ventricle (RV). When the RV contracts it pumps that blood to the lungs. Blood returning from the lungs moves into the left atrium (LA), and after LA contraction, is pumped into the left ventricle (LV), which then pumps it throughout the body. Four heart valves keep the blood flowing in the proper directions.
The electrical signal that drives this mechanical contraction starts in the sino-atrial node, a collection of specialized heart cells in the right atrium that automatically depolarize (change their voltage potential). This depolarization wave front passes across all the cells of both atria and results in atrial contraction. When the advancing wave front reaches the A-V node it is delayed so that the contracting atria have time to fill the ventricles. The depolarizing wave front then passes over the ventricles, causing them to contract and pump blood to the lungs and body. This electrical activity occurs approximately 72 times a minute in a normal individual and is called normal sinus rhythm.
The corresponding electrical signals identifying these events are usually referred to as the P, QRS (or R) and T waves or beats. More particularly, an atrial contraction is represented on an ECG by a P wave, a ventricular contraction is represented by an R wave and a ventricular repolarization is represented by a T wave. The atrium also repolarizes but this event (the U wave) is masked by activity in the ventricle and consequently it is not observable on an ECG.
Congestive heart failure is a condition that causes many deaths annually. The condition is characterized by weakness, breathlessness, abdominal discomfort, edema in the lungs and the lower portions of the body resulting from venous statis and reduced outflow of blood. These symptoms are associated with the inability of the heart to pump sufficient blood. Insufficiency may be associated with either the left ventricle, the right ventricle, or both. Cardiac output insufficiency may be caused by the failure of the heart to contract in an efficient way. If the physiologic conduction system has broken down, the chambers of the heart may not contract in a coordinated or effective manner. It is believed that cardiac efficiency could be improved by cardiac pacing that commences at or near physiologically optimum locations, or that can control or modify a cardiac wave front as the wave front passes through a chamber of the heart. In addition, dilated cardiomyopathy associated with heart failure often leads to a dysynchrony between the contraction of the left and right ventricles and to mitral regurgitation. Ventricular dysynchrony results in paradoxical septal wall motion and in reduction of cardiac output. Mitral regurgitation also results in a reduction in cardiac output. Both conditions increase myocardial strain that in turn leads to progression of the dilated cardiomyopathy via the expression of myocardial stretch proteins. Reduction in myocardial strain is thought to result in down regulation of these stretch proteins and a consequent slowing of the progression of or reversal of the dilated cardiomyopathy via reverse myocardial remodeling. Cardiac pacing to resynchronize ventricular contractions has been shown to increase cardiac output and reduce myocardial wall strain and it has been observed to produce reverse cardiac remodeling in human clinical studies. It is believed that cardiac pacing to directly control the contraction of the septal wall could also increase output, reduce mitral regurgitation, and reduce myocardial strain, leading to increased cardiac efficiency and potentially reverse remodeling. Cardiac pacing to modify the left ventricular base-to-apex activation sequence could also reduce mitral regurgitation, and again produce increased cardiac efficiency and potentially reverse remodeling.
Conventional pacemakers utilize a single or dual leads to apply pacing pulses. The dual (bipolar) lead typically includes a tip and a ring electrode. The lead is inserted in such a manner that the tip is imbedded into the cardiac muscle. A pacing pulse is then applied between the tip and the ring electrodes, thereby causing the cardiac muscle to contract. If a single unipolar electrode lead is used, the electric pulse is applied between the tip electrode and another electrode outside the heart, for example, the housing of the pacemaker. Bradycardia pacing therapy has usually been delivered through a pacing electrode implanted near the ventricular apex, that is, near the bottom of the heart. This location has been preferred not for physiologic reasons, but because most lead designs favor implantation at this site. A lead entering the right ventricle from the right atrium tends to extend into the lower apex of the ventricle where an active fixation apparatus, such as a helical corkscrew, may be used to secure the lead to the heart wall. Even if the distal tip of the lead is implanted at another location, it may be difficult or impossible to move the electrode to another location within the heart after initial implantation. The physician is thus limited to a single site for applying treatment. Bradycardia pacing therapy can be improved by delivering the stimulating pulse to a more efficient location than the ventricular apex. Studies have indicated that the abnormal contraction that results from apical pacing has long-term deleterious effects. Short-term studies using conventional pacing leads implanted in alternative locations have shown clinical improvements, but the long-term reliability of conventional pacing leads in these alternative locations is questionable and lead placement is difficult.
A single stimulating electrode, such as one available on a conventional lead, may not be implanted close enough to a physiologically preferred location in the patient""s heart to cause improved cardiac efficiency when the pacemaker stimulates the heart. In fact, stimulating at the bottom end of the ventricle may diminish cardiac efficiency as compared to a wave propagated from the top of the ventricle. Moreover, an apparatus with a single electrode cannot control cardiac contraction, guide the propagation of a wave front, force a selected path for a stimulating wave front, or create a coordinated simultaneous or near simultaneous cardiac contraction of large sections of the myocardium. Such controlled contractions may result in more efficient cardiac contraction, thereby reducing the overall demand on the heart, allowing the body to alleviate the symptoms associated with inefficient blood flow.
In view of the above disadvantages of the prior art, it is an objective of the present invention to provide an implantable cardiac stimulation system, such as a pacemaker, in which three or more electrodes are positioned in a chamber of the heart and an optimum electrode or electrodes are selected for pacing.
A further objective is to provide an implantable cardiac stimulation system with apparatus for shaping or modifying a propagating wave front, modifying the intrinsic ventricular cardiac activation sequence, or generating simultaneous or near simultaneous pacing pulses to the septum or right ventricular outflow tract during ventricular systole in order to improve left ventricular cardiac efficiency and reduce mitral regurgitation in patients with dilated cardiomyopathy.
Another object of the invention is to provide a cardiac stimulator system that uses multiple electrodes that can pace simultaneously or sequentially through any or all of the electrodes.
Other objectives and advantages of the invention shall become apparent from the following description.
Briefly, the subject invention pertains to an implantable cardiac stimulation system having a cardiac stimulator having electronic circuitry for the stimulation and a multi-electrode lead attached to the stimulator and inserted into one or more body cavities. (The term cardiac stimulator will be used herein to cover pacemakers as well as other cardiac devices such as internal cardioversion devices and defibrillators.) The lead is inserted into the cardiac cavity into a predetermined position. Alternatively the lead may be positioned in the veins, or it may be positioned externally of the heart. Since the lead has many electrodes, then an appropriate subset of electrodes is selected for stimulation.
More specifically, an implantable cardiac stimulation system is disclosed with a stimulator adapted to sense intrinsic cardiac activity and to generate a stimulation pulse or pulses responsive to intrinsic cardiac activity, said stimulation pulse or pulses having an amplitude associated with a stimulation threshold; and a plurality of implanted electrodes including at least one optimum electrode selected based on a physiologic parameter related to cardiac efficiency. Stimulation of the heart for a selected chamber usually begins at the optimum electrode or electrodes. Additional electrodes are implanted in a patient""s heart. These electrodes may be along a wave front propagation path, such that a wave front may be modified or reshaped by additional stimulation at selected electrodes, they may be at sites to be stimulated simultaneously or nearly simultaneously to cause a regional contraction of portions of the ventricular septum, or they may be positioned to induce a specific left ventricular activation sequence. Means are provided to identify the optimum electrode or set of electrodes and to identify the pattern in which cardiac tissue should be stimulated by the additional electrodes. Such means may involve stimulating pulses. A three dimensional map of electrode placement may be calculated. The relative locations of the electrodes may be determined by sensing an artificial wave front, emanating from a known electrode, such as the distal tip electrode or by field mapping techniques involving high frequency signals.
In a preferred embodiment, a lead having an elongated member is provided with the electrodes being formed on said elongated member. The electrodes comprise axially spaced electrodes disposed on said elongated member, each electrode being connected by a wire extending though said elongated member. The electrodes may be circumferential coils integral or continuous with the wires or may be rings connected to the wires by crimping or laser welding, for example. An electrode may also be provided at the distal end of the lead. The elongated member may be a tube housing the wires. The electrodes can be angularly spaced with respect to each about the elongated member. The tube may include an elongated cavity adapted to receive a removable stylet. The stylet may be more rigid then the lead and may be used for the implantation of the lead. After the lead is implanted, the stylet is removed.
In another aspect of the invention, a method is presented for treating congestive heart failure by implanting a lead having at least three electrodes, identifying an optimum electrode or electrodes for stimulation based on cardiac efficiency, and stimulating the heart through the optimum electrode or electrodes.